Silodosin, 1-(3-hydroxypropyl)-5-[(2R)-2-({2-[2-(2,2,2-trifluoroethoxy)phenoxy]ethyl}amino)propyl]-2,3-dihydro-1H-indole-7-carboxamide of Formula (I) is an indoline antidysuric which has a selectively inhibitory effect against urethra smooth muscle constriction, and decreases urethra internal pressure without great influence on blood pressure. Silodosin is available under trade names RAPAFLO® or UROREC®. Silodosin was first disclosed in EP 0600675 as a therapeutic agent for the treatment of dysuria associated with benign prostatic hyperplasia, where a process for producing the compound is also disclosed.

Since, Silodosin is an optically active compound having a complex chemical structure; its synthesis is relatively complex and requires a sequence of multiple steps.
U.S. Pat. No. 6,310,086, discloses a process for preparing Silodosin analogue compound from reaction of (R)-3-{5-(2-aminopropyl)-7-cyano-2,3-dihydro-1H-indol-1-yl}propylbenzoate with 2-(2-ethoxyphenoxy)ethyl methanesulfonate and finally isolated as a crude compound which is purified by column chromatography. The said process has a major disadvantage of using column chromatography which is not feasible at plant scale production.
PCT application no. WO 2012147019, discloses the preparation of Silodosin as shown in scheme-1, wherein the N,N-dialkyl impurity of Formula (IIa) formed during condensation of 3-{7-cyano-5-[(2R)-2-aminopropyl]-2,3-dihydro-1H-indol-1-yl}propyl benzoate of Formula (III) with 2-(2-(2,2,2-trifluoroethoxy)phenoxy)ethyl methanesulfonate of Formula (IV); is removed through preparation of monotartarate salt to give compound of Formula (VI). The compound of Formula (VI) is base hydrolyzed followed by cyano hydrolysis to give crude Silodosin of Formula (VIII) which is then further purified by crystallization to get desired pure Silodosin.

Major drawback of above said reaction process is that multiple isolations and crystallizations are required to get pure Silodosin.
Similarly, U.S. Pat. No. 7,834,193 discloses monooxalate salt represented by Formula VIa having 0.9% of dialkyl impurity represented by Formula IIa. The oxalate salt so obtained is subjected to alkaline hydrolysis followed by transformation of the nitrile to an amide.

Similarly, PCT application no. WO 2012147107, discloses the method wherein Silodosin is prepared by condensation of 3-{7-cyano-5-[(2R)-2-aminopropyl]-2,3-dihydro-1H-indol-1-yl}propyl benzoate with 2-[2-(2,2,2-trifluoroethoxy)phenoxy]ethyl methanesulfonate in solvent using base and phase transfer catalyst wherein, dialkyl impurity is formed up to 11%, followed by hydroxyl deprotection in protic solvent using base and phase transfer catalyst which is then subjected to purification to remove N,N-dialkyl impurity represented by Formula (IIb) up to 0.6% through the preparation of acetate salt. This process suffers from a serious drawback i.e., accountable formation of dialkyl impurity and even after purification the impurity is reduced to only up to 0.6%. Secondly, the process requires multiple isolations and purifications ensuing into time engulfing work-ups and purifications and hence incurring solvent wastage. This makes process lengthy, uneconomical and tedious to be performed at plant scale.
Another PCT application no. WO 2012131710, discloses the preparation of Silodosin in which the chiral compound (3-(5-((R)-2-aminopropyl)-7-cyanoindolin-1-yl)propyl benzoate) is reacted with 2-[2-(2,2,2-trifluoroethoxy)phenoxy]ethyl methane sulfonate in isopropyl alcohol using sodium carbonate as base. The reaction is completed in 40-50 h and about 9-11% of dimer is formed during condensation. After completion of reaction, it is subjected to hydroxyl deprotection and the crude compound so obtained is purified to remove the N,N-dialkyl impurity of Formula (IIb). The pure compound is then reacted with hydrogen peroxide in dimethyl sulfoxide to give Silodosin. The major drawback of this process is that the process is a multistep process wherein the condensation reaction is long-drawn-out resulting into countable amount of dimer formation during the process.
Thus, the prior art methods of preparing Silodosin require multiple and repeated purifications to synthesize DMF (Drug Master File) grade Silodosin. None of the prior art produces compound of Formula (VI) or (VII) with N,N-dialkyl impurity of Formula (IIa) or (IIb) in an amount less than 0.6% to 0.5% even after purification. Therefore to prepare highly pure Silodosin, there is a need to explore new synthetic schemes that could be more economical and scalable. The present invention provides a novel, improved, commercially viable and industrially advantageous process for the synthesis of Silodosin and its pharmaceutically acceptable salts or solvates thereof. The present invention focus on preparation of highly pure Silodosin in appreciable yields with minimal use of solvents wherein the Silodosin is isolated with purity ≥99.5% having N,N-dialkyl impurity less than 0.03% and other individual impurities below 0.1%.